Control for screen exciting oscillator



April 28, 1953 3 3, H D 11, r 2,637,005

I CONTROL FOR SCREEN EXCITING OSCILLATOR Filed Jan. 4. 1952 2 SHEETSSHEET 1 OUTPUT HIGH VOLTAGE OSCILLA TOR- AMPL lF/ER Ihventors:

Charles N Hood,11, William S.Oakes,J1-.,

Their Attorney.

April 28, 1953 c. N. HOOD 11, ETAL Filed Jan. 4. 1952 2 SHEETS-SHEET 2 Fig. 2.

M /o 7 1 I MASTER gmfi POWER OSCILLATOR AMPLIFIER AMPLIFIER PHOTOE'LECTR/C AMPLIFIER PHOTOELE'CTR/C L/GHT CELL 1 1 5m POWER 4 AMPL/F/ER fi': Z8 25 Z/a VAR/ABLE GAIN 7 22 AMPLIFIER In en t 0T 5 Charl es N. Hood,11, William 5. Cakes, Jr'z,

MASTER OSCILLATOR Their Attcprn e9.

*atented Apr. 28, 1953 CONTROL FOR SCREEN EXCITING OSCILLATOR I Charles N. Hood II, Schenectady, and William S. Oakes,.Jr., Pattersonville, N. Y., assignors to General Electric Company, a corporation New York Application January 4,1952, Serial No. 264,942

Claims.

This invention relates to a control for a screen xciting oscillator, and in particular to an imroved control for regulating the intensity of .lumination of the fluorescent coating on the creen of cathode ray tubes.

Cathode ray tubes having fluorescent coatings re so constructed that upon proper electrical xcitation the screen will glow. In order for a ibe of this sort to have a uniform glow intensity nd hence a picture of even contrast, it is neces- 11') that the tube screen be uniformly coated with ac fluorescent material.

It has been found that visual observance under low conditions is a satisfactory method of checkig the uniformity of the fluorescent coating. or such a test, it has been found that approxiiately 20 foot-candles of light should be prouced by the glow surface, and this li ht intensity constant for tubes of all sizes and under all acuum conditions.

A suitable glow exciting device is an electrode.

'hich when positioned at the focal point of the ibe and when properly excited by a vacuum ibe power oscillator produces a corona disbarge, which under suitable vacuum and voltge conditions causes the entire fluorescent surices of the tube to glow with a uniform intensity. In a cathode ray tube assembly line, a station provided for checking the uniformity of the uorescent coating. At such a station there light appear tubes of the same size, not held at 1e same vacuum, or tubes of various sizes, e. g. D", 16" or 30". Both size of the tube and the acuum conditions within it affect the power re- .iired for a suitable glow. Accordinrly, it would ot be possible with any one setting on the test quipment to maintain at a fixed level the li ht itensity for tubes of various sizes and vacuums. It is an object of this invention, therefore, to rovide a new test apparatus or device wherein 1e light intensity at the fluorescent surface is laintained constant and uniform irrespective f the vacuum conditions and the size of the ibe.

It is a further object of this invention to pro- .de a new test device for fluorescent coatin s on ibes wherein the power output of an oscillator responsive primarily to the light intensity on ie face of the tube.

It is a still further object of this invention to rovide an improved light sensitive device for arying the power output of a vacuum tube scillator.

Further objects and advantages of this invenon will become apparent and the invention will be more clearly understood from the following description, referrin", to the accompanying drawings, and the features of novelty which characterize this invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

While for the purpose of clarity this description is limited to cathode ray tubes, it is obvious that this invention is applicable to any surface which is made fluorescent as a result of an electrical stimulus, and it is to be understood that the example of the cathode ray tube is not intended as a limitation. but rather has it been chosen merely as an illustration to bring forth more clearly the operating principles and features of this invention.

Briefly, this invention comprises an apparatus including a power oscillator for illuminatin: the screen of a cathode rav tube and a light sensitive unit operating in response to the light intensity at the tube screen to vary the voltage output of the power oscillator.

In the drawing, Fig. 1 is a front elevation, partly in section, of a cathode ray tube positioned in a test stand and schematically connected to an electric circuit for causing its fluorescent surface to glow; Fig. 2 is a block diagram showing a svstem for controlling the li' ht intensity at a fluorescent surface; Fi 3 is a simple schematic diagram showing in greater detail the circuit shown in the block dia ram of Fig. 2.

In the drawings, a cathode ray tube I is shown positioned in a test stand 2 having a vacuum outlet or aperture 3, a bottom sealing cork 4, and an electrode 5 havinr one end 6 positioned at the focal point of tube l. Electrode 5 is energized by a vacuum tube oscillator-amplifier I which in turn is responsive to a signal from a light sensitive device 8.

' Cathode ray tube I is of the conventional type having a curved face surface 9 coated on the inside with a fluorescent layer I0. Tube I is coneshaped and tapers down to an open neck II through which the tube can be evacuated.

Under a suitable voltage and vacuum condition, the electrically charged tip 6 of electrode 5 produces a corona dischar e at the focal point of the tube. The corona ionizes the gas remaining in the tube, and the fluorescent coating II) is made to glow uniformly over the whole curved surface 9.

Tube I is evacuated by positioning it in a flange l2, a portion of stand 2 which has a resilient ring l3 made of a rub-berlike material. Ring [3 is positioned at the upper end of holder into the stand 2 so that its neck l'l' passes over and encompasses the electrode 5. The electrode Since the bottom of stand 2 is sealed by? a rubber cork or stopper 4, a tube i. islowered 5 is so held that its elevation maybevaried: to:

position tip 6 at the focal pointoftubes of dif.-'

ferent sizes, or near the average focalpoint of several different size tubes.

Electrode 5 is then electrically connecteditoz'the power output coil of a suitable high voltage oscillator-amplifier 1, which can produceat'tip 6 a potential in the orderfof 30,000 to 50,000 volts R; M: S. at about .25 kilocycles; A .voltageofi'this. range will produce a glow dischargeat t the flue-- rescent' surface 10 0f a.30" television picture tube when it has been evacuated to' -40 microns.

In order to' comprehend the requirements of: the voltage range under varying tube sizes and; vacuum conditions, it should be understood that for a glow intensity of 20 foot-candles approximately'20-,000 voltsshould be impressed at the" focal point of; a 12-inch tube at. microns. 60 microns; for the same tube a potentialinzthe order'of 33,000volts might'bei required So"; too, if the' size' of the tube is increased while thevacuumconditions are constant; the glow volt-- agemust be increased: It" isto' be understood,

however, that in an assembly line usually itis'not feasible to p mp down all tubes to exactly the same vacuum. Consequently; for simplicity of;

control, itis desirable to beable to. vary the power output of the oscillator to: compensate for any vacuum variations: or'tube size variations;

To compensate for the variance? in vacuum; conditions, tube? size, and other possible factors; 1. 6.,

. Sensitivity of the type of phosphor I0,

. Aluminized backing of: phosphor l0,

. Water molecules in the residual-gas,-

. Temperature effect-onionizationof the: resi.-

dualrgas,

5. Light conductivity oftubeface. 9,.

the voltage output at the tip of 6 of electrode 5 must be increased as either the vacuum o'r'tubei size is increased. This invention, therefore, re

lates to a unique device which. automatically.

compensates the voltage potential at the tip 6" of the electrode 5 in response to the light intensity at the face of the cathode ray tube. l

The primary elements of. thisdevice are schematicallyshown in the block'diagram o'fFig; 2 wherein it is indicated that a master signal is produced by the master oscillator M, which signal is applied to a variable. gain. amplifier. Photoelectric cell 8 actuates a photoelectric amplifier [5' which produces a signal that modifies the gain ofthe variable gain amplifier and, consequently; the signal from the variablegain amplifier which. controls the output of a p'oWerain-- While. we: do": not: intend to: limit ourselves to any particular circuit for varying the power' oscillator output-inresponsestosthe-light intensity at the fluorescent screenofithecathode ray tube; one simple circuit. is shown schematically-= iniFig;

3 wherein the master oscillator I4? comprises-:2}: 73'

resonant circuit l1 coupled to the input of a tub 18 which generates in the plate circuit thereo: a high frequency constant amplitude alternating current voltage. This voltage is impressec on a grid 20 of the variable gain amplifier tube It by conductor t9: v

The variable gain amplifier in response to th voltage of conductor l9 impresses an amplifier high frequency voltage on conductor 2| which i: connected to the grid 22 of the power amplifiei tub'ell'to operate the same; i. e., power amplifiei T produces a voltage proportional to the signa on conductor 21. which is connected to the grin 22 1' The direct current component of voltage ir the output circuit of tube I6 is prevented fron controlling the conductivity of the amplifier 2: by; a suitable blocking capacitor 2 la. The outpui of power amplifier 1 is impressed on the electrodr 6 by means of an output transformer 23 having the secondary winding tuned to the operating frequency of". the?- oscillator. [41. The output 01 plate: circuit: ofthe amplifier? T is" completec through; the primary"winding:of transformer 2i and a suitable:plate-supply 25..

From the foregoing. description, it is apparen1 thatthe electrodesB is energized by a high frequencyvoltage having an amplitude dependeru upon thegainofthe' variable gain amplifier it In} accordance withan important aspect of our invention, the. gain-of. amplifier 16 is automatically controllediin'. response'to the intensity o1 illumination at the face of'the cathode ray tubs t: to maintaizr that light intensityat a c-onstan-l pr'edeterminedvalue. In. accordance with the illustrated embodiment: of our invention, this control isfac'co-mplis'hedi'by energizing a seconc' control: grid zswrzth-e variabl ngain amplifier it in: accordance with: theamplified output of the photoelectric: device 81 The photoelectric device is coupled to the control member 28 0f the photoelectric amplifierr l5: to provide with resistor 21 avoltage'di viding circuit'connected across the plate. supply batte1iy-28ofi the amplifier it. As the intensity of illumination at the cathode ray tubescreen-r increases, the resulting in-cr-easec' conductivity of the: photoelectric device 8' render: the control memberzfi of amplifier l5 more positive, The resulting:increaseinthe conductivitg, of. amplifier l5 renders the platevthereof more negative. Since: the control member 25 of the amplifier i5 is coupleddirectly to the plate of amplifier I5, themore negative voltage of the plate. reducestheigain of. the:amplifier H5 with a resulting decrease in the? power. supplied by the master oscillator tothe-electrode 6. Converseh adecrease inthelight: intensity at the surface 01 the cathode ray tubedecreasesttheconductivityoi amplifier. I 5 renders the'plate'thereof more, positive, which in: turn; impresses. ai-more" positive voltage on thecontrol'memben2550f the variable gain amplifier to. increase: the powersupplied tc the electrode 5.

This equipment is such; that irrespective. 01 conditions within the tube to be tested, the lighi intensity automatically'adjusts-itself so: that approximately 20- foot-candles: of light: appear or the surface W of tube l It is.- obvious; that; this improwedi equipment i: a great feature in assuring proper. quality controll of? the; fluorescent: coating on. cathode ray tubes; It. assures,. automatically; a; uniform t'ubc illumination in such a manner'that thespeed? 01 inspection can be greatly increased.

0f: incidental: interest. is the fact. thatabattel-y 3|}: and. a manually controlled rheost'at- 31 can be connected through double-throw switch 32 to the grid 25 of the variable gain amplifier to impress a voltage on it for calibration purposes, or manual control of tube illumination.

Modifications of this invention will occur to those skilled in the art, and it is desired to be understood, therefore, that this invention is not intended to be limited to the particular embodiment disclosed, but rather is it intended to cover all modifications which are within the true spirit and scope of this invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A test stand for checking the uniformity of a fluorescent coating on the inner surface of a cathode ray tube comprising means for supporting a cathode ray tube, means for evacuating said tube, means for illuminating the face of said tube and control means for regulating the degree of power supply to said illuminating means, said illuminating means comprising an electrode having a tip at the focal point of said cathode ray tube, a power oscillator for energizing said electrode, said control circuit comprising a photoelectric tube positioned to be responsive to the light intensity at the face of said cathode ray tube and a variable gain amplifier operable in response to a, signal from said photoelectric tube to vary the power output of said power oscillator, thereby to maintain uniform the light intensity of the face of said cathode ray tube.

2. A device for checking the uniformity of light intensity at the face of a cathode ray tube having a fluorescent coating on its inner surface, said device comprising an electrode having a tip positioned at the focal point of said tube, circuit means including a power amplifier for energizing said electrode and a variable gain amplifier for controlling the output of said power ampli- Her, a photoelectric tube positioned to be responsive to the degree of light intensity at the face of said cathode raytube, andcontrol means including a photoelectric amplifier operable in response to said photoelectric tube for changing :he output of said variable gain amplifier there- Jy to control the output of said power amplifier n the voltage at said electrode.

3. Apparatus for producing a predetermined ight intensity at the face of a cathode ray tube having a fluorescent screen on the inner surface thereof, said apparatus comprising an electrode, support means for supporting a cathode ray tube with said electrode extending into the interior thereof, circuit mean including a power oscillator for impressing a voltage of variable magnitude on said electrode, a photosensitive control means positioned to be responsive to the light intensity at the face of a cathode ray tube carried by said support means, and control means operable in response to said photosensitive control means for controlling said circuit means to maintain the voltage at said electrode at the magnitude required to produce said predetermined light intensity.

4. Apparatus for producing a predetermined light intensity on a fluorescent screen, said apparatus comprising an electrode for activating said screen to illuminate thesame, circuit means including a power oscillator for energizing said electrode, and control means for varying the degree of energization of said electrode.

5. Apparatus for producing a predetermined light intensity on a fluorescent screen, said apparatus comprising an electrode for activating said screen to illuminate the same, circuit means including a power oscillator for energizing said electrode, control means for varying the degree of energization of said electrode, said control means comprising a photosensitive element posi-- tioned to be responsive to the light intensity of said screen, and means operable by said photosensitive element for varying the output of said power oscillator inversely to the degree Of illumination of said screen.

CHARLES N. HOOD II. WILLIAM S. OAKES, JR.

References Cited in the fileof this patent UNITED STATES PATENTS Number Name Date 2,124,404 Schroter July 19, 1938 2,203,353 Goldmark June 4, 1940 2,395,099 Cage Feb. 19, 1M6 2,457,456 Flory Dec. 28, 1948 2,460,471 Schade Feb. 1, 1949 2,474,380 Simmon June 28, 1949 2,523,328 Ranks Sept. 26, 1950 2,582,822 Evans Jan. 15, 1952 

